With the high-speed development of digital technology, the digital multimedia has already been used in a wide range of fields. At present, prevalent multimedia data formats include Windows Media Audio (WMA), Windows Media Video (WMV), RealMedia (RM), MP3 and OGG etc. In order to protect their intellectual property rights (especially copyrights) represented by digital information, a multimedia managing mode named digital right managing is put forward, in which the multimedia digital information needs to be encrypted and must be decrypted before it can be played normally. Therefore, it is necessary to get the key stream relevant to the digital right managing multimedia before playing the multimedia digital information. Then, it is played through the player which is able to play the digital right managing multimedia.
In the course of decoding of the digital right managing multimedia, the files can be read in the form of basic unit according to the type of file systems. The basic unit may be Page with a length of 512 bytes, 1024 bytes or 2048 bytes etc. By the same token, the length of key “k” may be 16 bytes or 32 bytes etc. Take the sequential cipher (stream cipher) algorithm of symmetric cipher for example, generally key “k” is used to get a stochastic key stream k(i) (i=0, 1, 2, . . . , n, n is the filelength of plaintext or ciphertext) by continuous algorithm when the digital right managing multimedia is played. Then, the key stream is used to encrypt the plaintext or decrypt the ciphertext. The above-mentioned plaintext refers to the content before encryption, while the ciphertext means the content after encryption. Furthermore, the application of ciphertext includes the content of digital right managing multimedia.
The digital right managing multimedia mechanism has well protected the intellectual property rights of copyright owner, but it also has some weak points in the course of playing the multimedia. The user needs to carry out decryption and get the original multimedia content before triggering the operation of fast-forward or fast-reverse (the number of fast-forward or fast-reverse may be the positive integer times of the number of Page). Therefore, in the course of fast-forward or fast-reverse, it is necessary to obtain the key relevant to each Page, so as to acquire a continuous key stream. Only when each key in the key stream is executed to decrypt each related Page of file, can the position of fast-forward or fast-reverse be acquired, and can the operation of fast-forward or fast-reverse be completed. The process of fast-forward or fast-reverse in itself will result in a great deal of operations (especially in the course of large fast-forward or fast-reverse), which would greatly lengthen the response time of media player as well as the course of fast-forward or fast-reverse, and lead to a waste of power supply for media player (especially the portable player whose battery has a short service life). By the same token, the waiting time is too long to the users. In reality, the user often makes use of the operation of fast-forward or fast-reverse when playing the media. The processing speed of processor or decoding chip in portable player is quite limited, so the operation of fast-forward or fast-reverse will give a great effect on the performance of portable player.